Reflection light barrier apparatus capable of recognizing strongly reflecting objects

Abstract

 A method and an apparatus for recognizing strongly reflecting objects in an autocollimation light barrier makes use of polarizing filters with crossed planes of polarization (16,17) at the light transmitter and light receiver of the barrier, and of a retro-reflector (12) with a depolarizing or polarization rotating characteristic. Thus the light transmitter directs light polarized in one plane to a retro-reflector across a region to be monitored and the retro-reflector produces depolarization, or rotates the plane of polarization of the light, prior to passing it to the light receiver. The light receiver will only recognize light whose plane of polarization has been appropriately rotated and thus will recognize light from the retro-reflector but not light reflected from a reflecting object (27) which interrupts the light barrier. The reflecting object would otherwise not be recognized as it would simulate the retro-reflector. Various polarizing arrangements are described.

Description

[0001] The invention relates to a method and to an apparatus for recognizing strongly reflecting objects in an autocollimation light barrier with a light transmitter receiver disposed on one side of the monitored region and a retro-reflector on the other side thereof.

[0002] In autocollimation light barriers of this kind the light transmitter and light receiver are united in a housing. The separation of the transmitted and received light beam can for example take place by means of a beam splitting mirror so that one and the same objective is used for the transmitted and received beams. It is however also possible to use two objectives arranged directly alongside one another for the transmitted and received light as the retro-reflectors which cooperate with the light transmitter/receiver do not exactly reflect an incident light beam back on itself but rather reflect it back within a narrowly defined solid angle. Thus, a relatively large fraction of the transmitted light beam from the transmitting objective (lens) will be deflected to a receiving objective arranged directly alongside it.

[0003] The known autocollimation light barriers operate such that the transmitted light beam is reflected from the retro-reflector to the receiver where the reflected beam is received by a photoelectric converter and converted into an electrical signal which is applied to an electronic processing unit. The processing unit delivers at its output, in the simplest case an electrical, optical or acoustic announcement as to whether the light beam passes freely between the light transmitter receiver and the reflector or whether it is interrupted by an obstacle. The advantages of a light barrier arrangement in which the transmitter and receiver are contained in one housing reside in the facts that an electrical connection is only necessary at one point and that it is only necessary to provide a reflector, the adjustment of which is relatively uncritical, at the opposite end of the monitored region. It is however frequently necessary to forego these advantages if it is also intended to recognize strongly reflecting objects within the monitored region. The reason for this is attributed to the fact that the transmitted light beam is not only reflected at the reflector but also at the object to be recognized. The receiver therefore still receives light when a reflecting object enters the monitored region so that in this case the presence of the obstacle within the monitored region is not signalled. As a consequence the problem which underlies the invention is to provide a method and an autocollimation light barrier of the kind previously named by means of which strongly reflecting objects within the monitored region can also be recognized.

[0004] For solving this problem the invention envisages that

a) linearly polarized transmitted light is used,

b) the state of polarization of the transmitted light is changed at the retro-reflector and

c) the light received is polarized prior to incidence on the photoelectric converter in a plane at 90° to the plane of polarization of the transmitted light.

[0005] In this connection the transmitted light can be at least partially depolarized at the retro-reflector or the plane of polarization of the transmitted Light can be rotated at the retro-reflector through an angle in the range between 10° and 170°, preferably in the range between 80 and 100°, and especially through an angle of 90°.

[0006] The thought underlying the invention is thus to be seen in the fact that the passive retro-reflector is made an active reflector so that it influences the linearly polarized light coming from the transmitter in a characteristic manner namely depolarizing it or rotating the plane of its polarization. The linearly polarized' light transmitted from the transmitter is thus only recognized by the receiver as light when it is reflected from the active retro-reflector. Thus, the depolarizing,or plane of polarization rotating characteristic of the reflector ensures that the linearly polarized transmitted light which is incident on the retro-reflector is reflected as unpolarized light or light the plane of polarization of which has been turned through 90°. The receiver is then able to recognize that component which has a plane of oscillation which is permitted to reach the photoelectric receiver. This effect of depolarization or rotation of the plane of polarization does not however occur for the previously disturbing surface reflections from strongly reflecting objects so that the original plane of oscillation of the transmitted light remains unchanged. The received radiation with this plane of oscillation is however not recognized at the receiver because of the 90° difference between the planes of polarization so that in this case no output signal appears at the photoelectric converter.

[0007] The invention thus provides a significant increase in contrast between radiation coming from the reflector and radiation originating from surface reflection at the strongly reflecting object.

[0008] The invention will now be described by way of example only and with reference to the accompanying drawing the single figure of which schematically illustrates an autocollimation light barrier in accordance with the invention.

[0009] The housing 18 of the light transmitter receiver unit includes a power supply 19 which can be directly supplied from the mains and which feeds a radiation source 20 in particular a gallium arsenide diode and an electronic processing unit 21. The light radiated from the radiation source 20 is formed into a sharply defined light barrier transmitted ray 14 via a condensor system,which, for the purposes of simplicity is not shown but is wellknown per se, and a front objective lens 22..The transmitted beam 14 passes through the monitored region 23 at the end of which there is located a retro-reflector 12 which reflects the transmitted light beam 14 back towards the transmitter receiver unit through a narrowly defined solid angle. In this manner a received beam of light selected from this solid angle beam reaches a further front objective lens 24 which is arranged directly alongside the front objective lens 22 of the transmitter and which concentrates the received light onto a photoelectric converter 25. The photoelectric converter 25 is connected to the electronic processing unit which provides a recognition signal 26 at its output depending on whether or not the quantity of light received by the photoelectric converter 25 is above a predetermined threshold.

[0010] In accordance with the invention a polarizer 16 is arranged between the front objective lens 22 and the radiation source 20 which linearly polarizes the transmitted light beam 14. The plane of polarization is preferably at the location of the smallest aperture of the rays within the apparatus.

[0011] As, in accordance with the invention, the retro-reflector 12 is of the kind which has a significant capability for producing depolarzation the received light beam 15 which reaches the front objective lens of the receiver is at least extensively depolarized or the plane of polarization is rotated through 90° relative to the plane of polarization of the transmitted beam 14. In accordance with the invention a further polarizer 17 is arranged between the front objective 24 and the photoelectric converter 25 which only allows the through passage of light the oscillations or vibrations of which take place in a plane which is rotated through 90° relative to the plane of oscillation of the transmitted light beam 14. Thus if the monitored region 23 is free the part of the light beam 15 which has a-plane of oscillation with a component in the direction of the transmission plane of the polarizer 17 is transmitted to the photoelectric converter 25.

[0012] If now an object 27 illustrated in broken lines and having a shiny surface enters the transmitted light beam 14 then light 28, which is likewise only shown in broken lines, will be reflected from its surface to the light receiver 13 within the housing 18. As however simple mirror surfaces do not at least significantly depolarize incident light the light beam 28 is linearly polarized in the same plane as the light beam 14 from the light transmitter 11. This plane of oscillation is at right angles to that which is transmitted by the polarizer 17 so that in this case no received light reaches the photoelectric converter 25 and the processing unit 25 thus signals the presence of an object in the monitored region 23.

[0013] Preferably, in accordance with the present invention, a retro-reflector 12 is used which rotates the plane of polarization of the transmitted light by 90°. In place of this an optical element 10 which rotates the plane of polarization of the incoming light through 45° can be placed directly in front of the retro-reflector 12. On its first passage through the element 10 the plane of polarization of the transmitted light ray is turned through 45° and on its second passage following reflection it is turned through a further 45^o thus making a total of 90°. The retro-reflector should in this case have characteristics which result in no or at most only trivial rotation of the plane of polarization of light reflected from the retro-reflector and should as far as possible also not preduce depolarization. In this way the best light yield is achieved at the receiver. In this case a dome lens is suitable as the retro-reflector but should however be armed with silicate.glass lenses in order to avoid the depolarization which prevails with transparent synthetic parts. An arrangement of several dome lenses in one plane can also be considered.

[0014] Reflecting arrangements of glass triples or glass Beck- prisms are themselves able to rotate the plane of polarization of incident light so-that in this case the arrangement of an optical element 10 for rotating the plane of polarization can be spared.

[0015] For achieving a depolarization effect arrangements of synthetic triple mirrors can be used which additionally bring about a certain rotation of the plane of polarization

[0016] Synthetic parts generally show a pronounced depolarization effect on account of their internal stresses.

[0017] If the retro-reflector 12 has no, or only trivial, depolarizing characteristics then a depolarizing optical element can be arranged at the position 10ifor example a plexiglass disc which is maintained in a state of pronounced internal stress.

[0018] Various cristals and foils are available for producing a rotation of the plane of polarization at the element 10. Scotchlite type foils can be manufactured both with and without depolarization effects. A good depolarization effect is shown by the Scotchlite foils having the designation "diamond-grade".

[0019] The polarization filters 16 and 17 are shown in the drawing arranged behind the front lenses 22, 24 as non-depolarizing silicate glass is used for the lenses. The polarization filter 16 should however be arranged in front of the front lens 22 if this is made of synthetic material as synthetic lenses frequently work in a depolarizing manner on account of their internal stresses. The polarizer 17 should likewise also be arranged in front of the lens 24 if the latter is made of a transparent synthetic material.

[0020] The dome lenses referred to earlier in this application are well known in the art as retro-reflectors and can be used either singly or in the form of a screen of lenses. Dome lenses are for example shown in DE-AS 25 18 828.

[0021] It will be appreciated by those skilled in the art that further modifications may be made to the apparatus herein disclosed without departing from the scope of the present teaching;in particular,althongh 90 is the preferred angle, other angles close to 90° could also be used.

Claims

1. A method of recognizing strongly reflecting objects(27) in an autocollimation light barrier with a light transmitter receiver(18)on one side of the monitored region(23)and a retro-reflector(12)on the other side thereof, the method being characterized in that

a) linearly polarized transmitted light (14) is used,

b) the state of polarization of the transmitted light (14) is changed at the retro-reflector (12) and

c) the light received (15) is polarized prior to incidence on the photoelectric converter (25) in a plane at 90° to the plane of polarization of the transmitted light (14).

2. A method according to claim 1 and characterized in that the transmitted light (14) is at least partially depolarized at the retro-reflector (12).

3. A method according to claim 1 and characterized in that the plane of polarization of the transmitted light is rotated at the retro-reflector (12) through an angle in the range between 10° and 170°, preferably in the range between 80° and 100 and especially by 90°.

4. Autocollimation light barrier for carrying out the method of claim 1 and characterized in that means are provided at the retro-reflector (12) for changing the state of polarization of the transmitted light (14).

5. Autocollimation light barrier according to claim 4 and characterized in that the retro-reflector (12) has a capability of producing depolarization.

6. Autocollimation light barrier according to claim 4 and characterized in that the retro-reflector (12) has the capability of producing a rotation of the plane of polarization.

7. Autocollimation light barrier: according to claim 4 and characterized in that optical elements (10) for rotating the plane of polarization are arranged in front of the retro-reflector (12).

8. Autocollimation light barrier according to claim 5 and characterized in that the retro-reflector is chosen from the group comprising synthetic triple mirrors or depolarizing Scotchlite foils.

9. Autocollimation light barrier according to claim 6 and characterized in that an arrangement of glass triple mirrors is used for the retro-reflector.

10. Autocollimation light barrier according to claim 7 and characterized in that a dome lens with an optical element (10) for rotating the plane of polarization arranged in front of the dome lens,is used as the retro-reflector.

Drawing